As is well known, a pixel refers to a picture element in digital image data. The pixel includes data that indicates the color or intensity of light, or some combination, at an undivided spot in the image. As used herein, the term pixel also refers to the smallest photosensor component that determines the color or intensity or both of light (photons) striking the sensor.
Very wide area applications such as persistent surveillance need very large images (100's to 1000's of Megapixels, Mpixels, 1 Mpixel=106 pixels) at high revisit rates (1 to 30 Hz). A frame rate indicates how quickly a new image of the same size as a focal plane array can be collected. A revisit rate indicates how quickly a new or composite image of the total scene under surveillance is collected. Historically, linear arrays using relatively small number of pixels have been scanned across a scene to reproduce the image. More recently two-dimensional arrays of pixels, known as staring arrays, have been used in a step stare mode where the image is produced by stitching together smaller images from a patchwork of snapshots as a quilt is stitched together from cloth patches.
A useful metric for the quality of imaging at long distances is the sensitivity, which is typically measured as the noise-equivalent-delta-temperature or NEAT, which is defined as the temperature signal change that is equivalent to the noise of the detector. For high sensitivity, a high fraction of the photons must be converted into electrical signal, and while the signal increases linearly with flux, the shot noise increases like the square root of the flux so that the sensitivity for background-limited images improves like the square root of the total integrated signal. The total integrated signal indicates a total number of photons collected over an integration time interval. Thus longer integration time intervals can increase the signal to noise ratio and provide higher sensitivity. However, in order to avoid a blurred or smeared image, the integration time for scanning arrays is limited to the time it takes motion of the image to cover approximately one-tenth of a pixel. So the integration times cannot be made arbitrarily long. For wide area imaging, at acceptable revisit rates, the integration time becomes very short. Thus, the sensitivity becomes poor.
One solution is to introduce multiple lines (banks) of linear arrays and then add the signal from each successive line with an appropriate time delay to produce the desired total integrated signal and sensitivity without image blur. Known in the art as Time-Delay-and-Integrate pixels (TDI), the approach has extended the capability of scanning imaging systems at the expense of increasing the complexity of a readout integrated circuit and optical system stability control. Advanced scanning imaging systems using TDI banks are disclosed by Goodnough and Tener in “Multi-bank TDI Approach for High-Sensitivity Scanners,” filed on Aug. 22, 2008, and published as U.S. Publication No. 2010/0046853 on Feb. 25, 2010, the entire contents of which are hereby incorporated by reference, except for terminology inconsistent with that used herein. Though very high-speed scanning technology is possible (>30 kHz sampling rates for each line of sensors), as the area to be imaged becomes very large, the noise of the amplifier in each readout unit cell can become significant relative to the very small optical signal produced in the short integration time required to avoid blur. Thus the imaging system sensitivity becomes degraded. In addition, for wide-angle scanned imaging, as the rate becomes high and more TDI banks are required, it is often difficult to achieve good optical image flow regions aligned with the pixel array to avoid image blur.
To provide wide area coverage using two-dimensional (staring) focal plane arrays, the array must be pointed, e.g., by mounting to a gimbal or moving a platform to which the optics are fixed, then maintained in stable position relative to the scene while the signal is integrated. If the platform to which the gimbal is mounted is moving, the gimbal must turn to keep the scene steady (e.g., stabilized within a fraction of a pixel) during the integration time interval. After the integration time interval ends, the system should be stepped to point to an adjacent location for another snap-shot.
The stepping can be achieved by various mechanical means, such as stepping the gimbal itself. Alternatively one can introduce a step-stare mirror which would typically slew (turn or slide quickly) and settle faster than the gimbal due to its lower mass. Another approach is to continuously scan the image and introduce a back-scan mirror in the optical path which stabilizes the image during integration and then returns to the starting position before the next integration.
In all such step-stare approaches, to avoid optical blur, it is advantageous if the image is stable to one tenth of a pixel or less during integration. Such step-stare mirrors must also be very flat and stiff to avoid image distortion. However, such flat stiff mirrors generally have greater mass than smaller or less stiff counterparts. The increased mass limits the rate such mirrors can be accelerated, decelerated and stabilized. Examples of advanced step-staring imaging systems are disclosed by Kasunic, Goodnough and Donohue in “Interlaced Focal Plane Array for Wide Area Surveillance,” filed on Sep. 9, 2011, and issued as U.S. Publication No. 2012/0081511 on Apr. 5, 2012, the entire contents of which are hereby incorporated by reference, except for terminology inconsistent with that used herein. While able to cover very large areas, the revisit rates are limited by the step-stare slew-and-settle times; and, the step-stare mechanisms introduce optical imaging system complexity and cost.
One simplistic approach is to use a very large two-dimensional array of pixels with commensurately sized optical system. Such extremely large staring FPAs and optics can drive system costs beyond the limits of what can practically be put into the field. It was recognized that smaller staring FPAs that can be stepped over a scene (thus creating a high Ground-Pixel/Physical-Pixel or GP/PP ratio) can dramatically lower cost and SWaP (Size, Weight and Power) of such systems. However, such small staring arrays put more stringent constraints on the support structures and optical components used with such arrays. For example, all elements, such as a lens and a stepping mirror, in the optical path must maintain uniformity and consistency over the entire optical wavefront or there will be image distortion. Stepping mirrors must then be very stiff, increasing the mass and limiting the ability to quickly point, smoothly rotate, and then quickly point again (motion dynamics).
Furthermore, the stepping mechanism often involves large stepping mirrors that grow in size with higher altitudes and lower ground-sample-distance (GSD), which involves a smaller space between pixels on the ground. Lower GSD involves a larger focal length and, thus, a larger beam, and, thus, a larger mirror. Because the mirror size is roughly proportional to aperture size, and the inertia of these mirrors can grow as fast as the cube of its radius from the pivot, the stepping mechanism is often the key limit in image size and revisit times. Other optical chain mechanisms can be stepped but they similarly scale with aperture size, are inherently heavy, and/or have more limited angular step range.